In general, extruders are frequently used in the granulation of plastics. These extruders press molten plastic raw material through nozzles of a perforated plate into a coolant, e.g. water. In this process, the material emerging through the openings of the nozzles is cut by a cutter arrangement with at least one rotating blade to produce pellets. Corresponding devices, which carry out methods for underwater granulation, are known as underwater granulators such as those made by SPHERO® from the firm Automatik Plastics Machinery GmbH.
The coolant, such as water, which is provided in such underwater granulators for cooling and transporting the freshly cut pellets or granules, is fed to an underwater granulator through an inlet that typically is offset from the axis of rotation of the cutter arrangement and correspondingly offset from the intended axis of symmetry of a housing of such an underwater granulator, and is discharged together with the pellets located therein through an outlet that generally is radially placed. Even though the geometric arrangement and/or dimensioning of such devices, in particular the inlets and outlets and housings of such devices, are chosen such that a certain swirl is imparted to the coolant in the housing of such devices so that it presents less resistance to the rotary motion of the cutter arrangement. Nonetheless, strong eddies and turbulence in the interior of the housing of the produced cutting chamber result from the rotary motion of the cutter arrangement, which has heretofore necessitated a relatively high drive power needed to drive the rotating cutter arrangement.
The aforementioned eddies and turbulence usually arise through the action of the cutter arrangement on the coolant located in this area of the cutter arrangement, since the coolant is accelerated centrifugally by the rotating cutter arrangement. Even at low speeds, the shape, number, and rotary speed of at least one blade have the effect that more cooling fluid is displaced and pumped away from the region of the at least one blade or in the region between multiple blades than the total throughput of cooling fluid in this region.
The cooling fluid that is “missing” in this region is therefore drawn in again from a region in the housing or in the cutting chamber that faces away from the perforated plate and is thus opposite thereto. In this process, the cooling fluid is typically forced into a toroidal path, which is undesirable on account of the high relative speeds and the associated frictional power dissipation in the cooling fluid. This power dissipation must be additionally supplied and compensated for by increased energy input through the drive of the at least one blade of the cutter arrangement.
The German published patent application DE 101 37 524 A1 describes a device for granulating thermoplastic materials emerging from nozzles in a perforated plate, wherein the nozzles, which are provided in a substantially circular arrangement, are swept by blades rotating about a blade carrier shaft. Said blades are held in an inclined position with respect to the radial direction by a blade carrier. The blade carrier shaft extends through the center point of the circular arrangement and a coolant is supplied to the perforated plate and to the blades for cooling and carrying away the pellets thus produced. Here, the blade carrier with blade carrier shaft is arranged in round housing extending to the nozzle plate. A coolant inlet discharge is located in the blade carrier shaft to discharge cooling fluid into the housing in the tangential direction to produce an annular flow. The blade carrier rotates in the housing. The cooling medium rotates in the housing with a speed and rotation direction corresponding to the rotational speed and rotation direction of flow openings in the blade carrier. The cooling medium reaches the blades preferably in an axial direction by way of the flow openings.
The resultant flow of the cooling fluid will be in a spiral running radially from the inside to the outside. The intent with this device is to provide a path for the flow of coolant in the device that allows uniform flow largely free from turbulence.
The document WO 2006/122340 A1 describes a device for granulation in which a cylindrical granulating housing is tightly closed at one end by a perforated plate with nozzle openings. A cooling fluid is supplied via an annular space surrounding the perforated plate, through a plurality of openings axially parallel to the rotational axis of the cutter arrangement. In the process, the cooling fluid washes away the pellets that are thrown radially outward by the separating process, axially parallel to the rotational axis, from the rotation region of the cutter arrangement. The coolant with the pellets contained therein is then discharged through an outlet in the region of the housing facing away from the perforated plate.
Therefore, a need exists for a method and a device for granulating thermoplastic material that allows improved flow control of the coolant and permits energy-efficient granulation in a relatively simple and economical way.
The present embodiments meet these needs.
The present embodiments are detailed below with reference to the listed Figures.